1. Field:
This invention relates to chemical gas sterilization. It is particularly directed to ethylene oxide sterilization and provides an improved method for controlling temperatures within a sterilization chamber at the initial stage of the cycle.
2. State of the Art:
Chemical sterilization using biocidal gases, notably ethylene oxide, is a well-established procedure in various fields of applied biological science, particularly in the health care professions. Standards have evolved for such procedures and have been published as American National Standard ST24 by the Association for the Advancement of Medical Instrumentation (AAMI) (ANSI/AAMI ST24-1987). Standard ST24 sets forth certain physical performance tests, including the following:
. . . 3.1.5.6 "Physical Performance Tests." (1) Temperature Control. After exposure pressure has been reached and exposure timing has begun, the chamber temperature shall stabilize within the first ten percent of the selected exposure time, after which the variation of temperature within the chamber may not exceed .+-.3.degree. C. (5.5.degree. F.) throughout the remaining exposure phase of the cycle . . . .
Ethylene oxide (EO) sterilization is often conducted in a water-jacketed pressure chamber. The sterilization process is initiated by evacuating the chamber to remove air. Moisture is introduced during a conditioning phase. The EO sterilant is then introduced to the chamber to a predetermined pressure, usually above atmospheric. As suggested by the physical performance test of Standard ST24, one of the most important process parameters for EO sterilization is temperature control. Typically, temperatures are controlled by introducing steam to a water-filled jacket surrounding the sterilization chamber to balance heat losses through the unjacketed ends of the chamber. The steam causes the temperature of the jacket, and thus the chamber, to rise. When a predetermined jacket temperature is reached, steam is no longer introduced to the jacket. In time, the jacket temperature tends to drop as heat is dissipated to the ambient environment. Additional steam is introduced intermittently to maintain the temperature within the chamber at the setpoint (or within the prescribed tolerance band above the setpoint).
While use of the water jacket and steam provides good temperature control within the standards set by ANSI/AAMI ST24-1987, that test is performed using an empty chamber and ignores the impact on a load experiencing the actual temperature and pressure conditions within the chamber during the first ten percent (10%) of the time elapsing after the exposure pressure has been reached and exposure timing has commenced. Many of the plastics and delicate electronics in current use rely upon ethylene oxide sterilization procedures because they cannot withstand alternative methods of sterilization. Such modern loads are relatively intolerant to high temperatures. Accordingly, it is of significant importance to ensure that these delicate instruments and implements be protected from extreme temperatures throughout their entire exposure and not only during the last ninety percent (90%) of the exposure period.
Inherent in the EO sterilization procedures currently practiced, is an uncontrolled temperature surge at the commencement of the sterilization cycle. When EO sterilant is compressed within the chamber to a predetermined pressure (the gas pressurization phase), significant heat of compression is released within the chamber. This heat of compression typically causes the chamber temperature to rise briefly, e.g., by as much as 12.degree. C. above the desired chamber temperature setpoint. The setpoint temperature is established within the chamber by heating the water jacket external of the chamber. Jacket heating is independent of the impact of any incidental heat of compression. Moreover, the aforementioned performance test is conducted at a time when the impact of the heat of compression is largely dissipated. Accordingly, under normal monitoring and test procedures, any impact of the heat of compression is ignored. Nevertheless, this impact is detrimental to the load subjected to the sterilization procedure.
Reliance upon ambient conditions to dissipate the heat of compression occurring in the gas pressurization phase of the sterilization procedure is inadequate. There remains a need in the art for an improved method whereby this heat of compression may be controlled sufficiently to safeguard delicate materials and instruments exposed to gas sterilization techniques.